Why does Gretel give up on her dolls and replace them with maps?

Gretel's crush on Lt. Kotler causes her to put away her dolls and pay more attention to the war. From the text, we can infer that she believes this will make her appear more grown up and provide a connection.

One afternoon a month or so earlier, around the time that Lieutenant Kotler had left Out-With, Gretel had decided that she didn't like dolls any more and had put them all into four large bags and thrown them away. In their place she had hung up maps of Europe that Father had given her, and every day she put little pins into them and moved the pins around constantly after consulting the daily newspaper.